[Android] AGP 9 / built-in Kotlin compatibility

## Summary

Flutter apps are adopting **AGP 9** with **`android.builtInKotlin=true`** and **`android.newDsl=true`**. Plugin Android modules that still apply the **legacy Kotlin Gradle Plugin** (`org.jetbrains.kotlin.android`) or rely on **pre–AGP 9 Gradle DSL** can break or conflict with host apps on the new toolchain.

## Ask

Please plan migration of `:permission_handler_android` (and related Android Gradle) per Flutter’s guide for **plugin authors**:

https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors

## Context

- Ecosystem: [flutter/flutter#184836](https://github.com/flutter/flutter/issues/184836)
- Existing requests ([#1513](https://github.com/Baseflow/flutter-permission-handler/issues/1513), [#1484](https://github.com/Baseflow/flutter-permission-handler/issues/1484)) are not scoped to **AGP 9 / built-in Kotlin / new DSL**; this issue is for that track.

Happy to help test with a prerelease.